Pruritus, or itch, is an uncomfortable skin sensation that provokes a desire to scratch. Although itch may be acute, for example, from an insect sting, chronic pruritus originates from many different causes. It is a seriously debilitating condition, comparable to chronic pain, which negatively impacts quality of life.
Chronic pruritus affects millions of people worldwide, although solid epidemiological data is very limited. For example, one study reported that 8-10% of the population of Oslo suffer from chronic pruritus from all causes (F. Dalgard et al., J. Investig. Dermatol. Symp. Proc., 2004, 9(2):120-5). Patients with certain diseases and conditions report high incidences of chronic itch, including those with psoriasis (78-84%), Hodgkin's disease (25-35%), dialysis patients (22%), and polycythaemica vera (48%) (M. Metz and S. Ständer, CME Dermatol., 2008; 3(3):124-143). Chronic pruritus is also a prevalent symptom in cutaneous T-cell lymphoma (68-93%), a disease that includes mycosis fungoides and Sézary syndrome (N. Meyer et al., Acta Derm. Venereol., 2010, 90:12-17). Pruritus is the most common dermatological complaint in elderly patients (S. Beauregard and B. A. Gilchrest, Arch. Dermatol., 1987, 123:1638-43). Itch is often the side effect of certain drugs, such as EGF receptor antagonists.
Antihistamines can sometimes effectively treat itch due to acute urticaria, but many chronic pruritic diseases respond poorly to conventional H1 receptor antagonists (Tey H. L. and G. Yosipovitch; Br. J. Dermatol., 2011, 165(1):5-17). In addition to marginal efficacy, antihistamines can also cause intolerable drowsiness. Other current therapies possess various limitations. For example, anticonvulsants such as gabapentin inhibit spinal mechanisms in the perception of itch, but their use is limited due to their slow onset of action (5-6 weeks) (Metz and Ständer, 2008). Opiate receptor antagonists such as naloxone, nalmefene, and naltrexone decreased pruritus symptoms in patients with liver and kidney disease, although significant central nervous and gastrointestinal side effects occurred (Metz and Ständer, 2008; N. V. Bergasa et al., Hepatology, 2006, 44(5):1317-23).
Substance P, the endogenous ligand for the neurokinin-1 (NK-1) receptor, is a significant mediator of pruritus (T. Andoh et al., J. Pharmacol. Exp. Ther., 1998, 286:1140-5). Intradermal injection of substance P elicits an itch sensation in human subjects, and an associated itch response in mice. The substance P-induced itch-associated response in mice is not inhibited by antihistamines (B. Amatya et al., Skin Pharmacol. Physiol., 2010; 23:133-138; C. Weidner et al., J. Invest. Dermatol., 2000, 115:1015-1020). In an experiment designed to study the role of substance P in pruritus, Ohmura et al. reported that tachykinin NK-1 receptor antagonist, BIIF 1149 CL, inhibited scratching behavior in a picrylchloride-induced dermatitis model in NC/Nga mice (Eur. J. Pharmacol., 2004, 491:191-194; U.S. Patent Application No. 2003/100565).
Aprepitant (Emend®), an NK-1 receptor antagonist, is approved by the FDA for use in the prevention of chemically induced nausea and vomiting (emesis) after chemotherapy. Duval and Dubertret first reported that oral aprepitant (80 mg daily) had utility in treating pruritus in three patients with Sézary syndrome (N. Engl. J. Med., 2009, 361(14):1415-6). Torres et al. disclosed similar results (J. Am. Acad. Dermatol., 2012; 66(1):e14-5). Ständer et al. conducted a small, open-label study which demonstrated that aprepitant significantly decreased chronic pruritus caused by conditions such as atopic diathesis and prurigo nodularis. In this study, twenty previously untreatable patients were given a daily dose of 80 mg for 3 to 13 days. Eighty percent of the patients experienced a considerable reduction in itch intensity (S. Ständer, et al., PLoS One, 2010, 5:6, e10968). However, Wallengren conducted a follow-up double-blind study based on Ständer's work testing a single dose of topical aprepitant blended at a 5% concentration in a lipophilic vehicle in patients suffering from chronic pruritus of various etiologies. Although the drug was absorbed into the skin, the patients' itch was not alleviated (J. Wallengren, Arch. Dermatol., 2012, 148(8):957-9).
Although oral aprepitant is generally well-tolerated, it is extremely expensive, limiting its use in chronic pruritus (Tey, 2011). Further, aprepitant is a moderate inhibitor as well as an inducer of CYP3A4 and CYP2C9, indicating that drug-drug interactions with chemotherapeutic agents and corticosteroids must be considered (Torres, 2012). Mir and Coriat have suggested that the risk of drug-drug interactions with aprepitant is high because it can alter the activity of cytochrome P450 3A4 isoform (CYP-3A4), an enzyme involved in the metabolism of a range of commonly prescribed drugs, including tyrosine-kinase inhibitors, either inducing or inhibiting the CYP-3A4, depending on which drugs are given concomitantly. Tyrosine-kinase inhibitors do not induce frequent nausea and emesis; therefore, clinical experience with concomitant administration of aprepitant and these drugs is scarce. Furthermore, the pharmacokinetics of tyrosine-kinase inhibitors varies widely between patients, and drug-drug interactions are common (O. Mir and R. Coriat, The Lancet, 2012, 13:964-965). Thus, the need for additional, safe treatments for acute and chronic pruritus exists.